Cladding pumped fibre lasers are becoming more common. In these types of lasers, a core of an optical fibre is doped with a rare earth element to become an active gain medium and pump radiation is applied via a cladding layer. The pump radiation is normally produced by semi-conductor laser diodes and a means is provided for coupling their input into the cladding of the rare earth doped fibre. Bragg gratings formed in the core of the optical fibre are employed as mirrors to form a laser cavity.
Co-pending UK Patent Applications Nos 0612463.0 and 0623452.0 describe schemes for coupling pump radiation into or out of an optical fibre and a cladding pumped fibre laser having a high degree of pump isolation. These disclosures are incorporated herein by reference.
Conventional lamp pump non-fibre lasers can be protected in various ways against back radiation and other types of damage. This is more difficult with fibre lasers.
Material processing a highly reflective material such as copper can result in a large amount, perhaps even up to 90% or so, of the power being reflected if the incident pump light is not coupled into the material properly. This may come as a result of the incident laser beam not having sufficient intensity either through lack of power or the laser output being out of focus. This reflected power can have serious consequences for the laser source if it is not controlled properly, either through damage of the delivery fibre and surrounding optics or delivery feedback into the source laser causing it to run unstably or causing the light to damage the sensitive pumped diodes. It will be recalled that pumping radiation from the diodes is applied through a cladding layer to the doped region and therefore any reflected radiation which finds its way back into the cladding layer could, in an unprotected system, find its way directly to the pump diodes where it can severely damage them.
Furthermore, it is well know (Optics letters Vol 13 No 9) that an optical fibre carrying only modest amounts of power densities can experience a process referred to as ‘the optical fuse effect’. The effect is initiated by a defect in the carrying optical fibre such as damage to an end face or core of the delivery fibre. The defect in the optical fibre creates a region of high absorption which can raise the core to above 1000 C and a plasma is created in the core. This plasma will propagate towards the source of radiation, destroying the core of the delivery fibre at a subsonic velocity. This propagation will continue until either the operation of the source of radiation is destroyed or there is an increase in the mode field diameter which reduces the intensity of both the optical signal and pressure wave, arresting the propagation of the optical discharge. This uncontrolled propagation of the optical discharge can propagate into vital components of the fibre laser resulting in its destruction.
The present invention arose in attempt to design a fibre laser system that is highly resistant to damage and is robust.
A further object of the invention is to provide a fibre laser system that includes electronic means for protecting the laser from damage caused by the optical fuse effect, such that operation of the laser can be restored by a simple process.